Method of anodically polishing copper



Patented Apr. 18, 1944 METHOD OE ANODICALLY POLISHING COPPER Charles L. Faust, Columbus, Ohio, assignor to Battelle Memorial Institute, Columbus, Ohio, a

corporation of Ohio Application November 6, 1939, Serial No. 302,951

(Cl. ZIM-140) 3 Claims.

This invention relates to a method of and electrolyte for anodically polishing copper. More particularly, the invention relates to an aqueous electrolyte comprising phosphoric acid, chromic acid and water and to the use of such electrolyte in the anodic polishing of copper.

Various compositions of electrolyte have heretofore been proposed for use in the anodic treatment of copper. I have now found, however, that an aqueous electrolyte comprising phosphoric acid, chromic acid and Water, as the essential ingredients, constitutes a very satisfactory electrolyte for use in the anodic polishing of copper. If the relative proportions of these ingredients of the bath be kept within certain limits, which I have determined, very satisfactory polishes can be produced on copper, while at the same time producing surface finishes having relatively superior characteristics to those obtainable by mechanical polishing or bulllng operations.

It is therefore an important object of my invention to provide an electrolyte containing phosphoric acid, chromic acid and water within certain limits as to their relative proportions, for use in the anodic polishing of copper to impart thereto a highly lustrous surface.

It is a further important object of this invention to provide a method of anodically polishing copper, using an electrolyte of novel composition that is operative throughout a wide range of anode current densities and temperatures.

Other and further important objects of this invention will become apparent from the disclosures in the specification and the accompany ing drawing.

This invention (in its preferred form) is illustrated in the drawing and hereinafter more fully described.

On the drawing:

The gure represents a triaxial diagram showing the relative proportions of phosphoric acid, chromic acid and water for compositions of the electrolytes coming within the scope of this invention.

In the accompanying diagram, the respective sides of the triangle indicate the percentages of phosphoric acid (H3PO4) of water (H2O), and of chromic acid (CrOa) from to 100%. On the basis of experimental data, I have determined the relative proportions of phosphoric acid, chromic acid and water that give compositions of electrolyte that are operative for the anodic polishing of copper, The arca representing operative compositions of electrolyte is dened on the accompanying triaxial diagram by the solid lines AB, BC, CD and DA. Within the area so defined, any composition selected will be found to be operative in the method hereinafter described for the anodic polishing of copper.

In order to get the best polishing results, however, I have found that the range of proportions of phosphoric acid, chromic acid and water should be kept Within somewhat narrower limits and these narrower limits are represented on the accompanying diagram by the area defined by the lines EB, BF and dot and dash line FE. The preferred compositions of electrolyte, with respect to the relative proportions of phosphoric acid, chromic acid and water, lie within this second lesser area, which is wholly enclosed within the broader area rst defined.

The reading of a, triaxial diagram such as the accompanying one is well understood but the following will be given for purposes of illustration. The point on the diagram represented by the letter A, for instance, indicates a composition comprising a small but significant proportion, say 0.1% of phosphoric acid, of chromic acid and the balance, or somewhat less than 35%, water; the point indicated by the reference letter B a composition comprising 85% of phosphoric acid. a small but signicant proportion, say 0.1% of chromic acid and the balance, or somewhat less than 15%, water; the point indicated by the reference letter C a composition comprising 10% of phosphoric acid, say 0.1% of chromic acid and the balance, or somewhat less than 90% of Water; the point represented by the letter D a composition comprising about 0.1% of phosphoric acid, 10% of chromic acid and the balance, or about 90% water; the point represented by the reference letter E :a composition comprising 50% of phosphoric acid, 26% of chromic acid and 24% of water, and the point represented bythe letter F a composition comprising about 20% of `phosphoric acid, say 0.1% of chromic acid and the balance, somewhat less than Water. The maximum phosphoric acid content is about and the minimum about 0.1%; the maxlmum chromic acid content 65% and the minimum about O.l%; and the maximum water content and the minimum about 13%. The preferred composition limits are from 20 to 85% phosphoric acid, from 0.1 to 39% CrOa, and from 13 to 80% water. It should be understood, however, that the relative percentages of these three ingredients are all inter-dependent, so that a point should be selected within the above described areas in order to insure that the composition of electrolyte be a satisfactory one.

While the triaxial diagram shows the relative proportions of phosphoric acid, chromic acid and water in a system consisting of only these three components, suitable baths of these three ingredients may also include other ingredients, such as other acids and/or salts. From the diagram, however, the relative proportions that phosphoric acid, chromic acid and water should bear to each other may be determined for operative and preferred ranges of composition, regardless of what other non-essential ingredients may be present in the bath.

Thus, for instance, in making up an electrolyte for the anodic polishing of copper, one might select the point X on the triaxial diagram as being a bath of preferred composition. The composition represented by the point X would be 60% of phosphoric acid, of chromic acid and 30% of water. During the continued use of such a bath in the electropolishing of copper, the bath composition would necessarily change, owing to the anodic dissolution into the bath of copper from the copper undergoing polishing. There might also be somechange in the water content of the bath, such as an increase due to absorption by the bath of moisture from the air, or a decrease due to the evaporation of water from the surface of the bath, to decomposition of water by electrolytic action, or to loss of water from the bath by entrainment thereof in gases given off from the bath.

Notwithstanding such changes in its composition as may occur during continued use, if the relative percentages of phosphoric acid, chromic acid and water, expressed as percentages by weight of the total Weight of just these three ingredients in the bath composition, remain within the area dened by the lines EB, BF and FE, the bath will continue to operate satisfactorily. Even if the bath composition be so altered during continued operation that it falls within the less preferred area defined by the lines AE, EF, FC, CD and DA on the accompanying diagram, the bath will continue to function, although not so satisfactorily.

Consequently, where the relative percentages of phosphoric acid, chromic acid and water in a given bath composition lie within either the preferred or less preferred areas defined on the accompanying triaxialdiagram, such bath composition is intended to come within the scope of my invention, even though it may contain other acids than phosphoric and chromic and even though it may contain a substantial quantity of metallic salts.

Instead of chromic acid, soluble chromates and bichromates may be substituted therefor and are to be considered the equivalent of chromic acid on a stoichiometric basis. The term chromic acid equivalent, as used in this specification and in the claims, is therefore intended to include chromic acid itself (CrOz) and stoichiometrically equivalent weights of soluble chromates and bichromates.

Similarly, in place of orthophosphoric acid, other phosphoric acids, such as metaand pyrophosphoric acids, may be used and are to be considered as included within the term phosphoric acid.

In the method of anodically polishing copper, using a bath of a composition indicated as suitable by reference to the accompanying triaxial diagram, the copper, or an article having a surface of copper, is made the anode in a bath of the selected composition and an electric current is passed therethrough of suiiicient density and for a sufficient length of time to produce the desired high degree of luster, or polish, on the metal surface. By employing an electrolyte having a composition within the preferred area dened on the accompanying triaxial diagram, a highly lustrous, mirror-like surface can be readily obtained. The highly lustrous surface obtainable by my method, using an electrolyte of preferred composition, is an important feature of my invention and one that sharply distinguishes it from the prior art finishes produced in the electrolytic cleaning of copper.

The formation of highly polished and lustrous surfaces is undoubtedly associated with the presence of a polarizing film over the surface of the metal during the process of anodic dissolution. 'I'he nature of this film is such that selective attack on the various phases present in the copper is minimized. Anodic dissolution apparently takes place at a. relatively high rate and at a high anodic polarization value, with the result that the anodic dissolution of the metal acts to level the crystal surfaces thereof and to produce a mirror-like nish. I'hese conditions do not prevail in the simple electrolytic cleaning treatment known to the prior art. -1 In order to obtain the best results in a reasonable length of time, it is preferable to use relatively high current densties, such as those of the order of magnitude of from to 1000 amperes per sq. ft, It will be understood, however, that lower current densities, even as low as 50 amperes per sq. ft., may be employed with consequent prolongation of the time of treatment. Higher current densities, up to as high as 2000 or 3000 amperes per sq. ft., may also be employed, but such' high current densities imply, in general, larger currents which require more expensive equipment. The length of time to effect the desired resuits depends upon the magnitude of the current densities employed and to some extent upon the particular characteristics of the copper to be polished, and also upon the character of the surface of the copper initially. Rough surfaces, of course, require a longer time to polish than relatively smooth ones.

With any of the compositions of electrolyte lying within the preferred area defined by the lines EB, BF and FE, on the accompanying triaxial diagram, excellent polishes are obtained on copper, when the metal is made the anode therein at current densities of 500 amperes per sq. ft. and the treatment carried out for a period of 1% to 12 minutes, employing a bath temperature lying within the range of 60 to 170 F. In general, the temperature may be maintained at any point between room temperature and the boiling point of water, but temperatures of around 100 F. are found very satisfactory.

From the foregoing description of my invention, it will be apparent that I have provided a novel composition of electrolyte and a method whereby the same may be used to produce highly lustrous polishes on copper. My electrolyte and method avoid the disadvantages of the old methods of mechanically polishing the metal, and enable the production of copper, and articles made therefrom, having surfaces that are free from mechanical strain, dragging and pi1ing, and which are superior to those obtained by mechanical polishing methods.

It will, of course, be understood that various details of the process may be varied through a wide range without departing from the principles of this invention and it ls therefore, not the purpose to limit the patent granted hereon otherwise than necessitated by the scope of the appended claims,`

I claim as my invention:

l. The method of anodically polishing copper, which comprises making the copper the anode in a solution comprising essentially from 0.1 to 85% of phosphoric acid, from 0.1 to 65% of CrOa, and from 13 to 90% of water, all of said percentages being by Weight of said solution and the relative percentages of said three solution ingredients lying within the area dened in the accompanying diagram by the line AB, the straight line BC, the straight line CD, and the straight line DA, and passing an electric current therethrough of sufiicient density and for a suiiicient period of time to effect the polishing of said copper.

2. The method of anodically polishing copper, which comprises making copper the anode in a solution comprising essentially from 20 to 85% of phosphoric acid, from 0.1 to 39% of CrOa, and from 13 to 80% of water, all of said percentages being by weight of said solution and the relative percentages of said three solution ingredients lying Within the area dened in the accompanying diagram by the line EB, the straight line BF, and the line FE, and passing an electric current therethrough of sufficient density and for a suicient period of time to effect the polishing of said copper.

3. The method oi anodically polishing copper, which comprises making copper the anode in a solution comprising essentially from 20 to 85% of phosphoric acid, from 0.1 to 39% of CrOs, and from 13 to 80% of water, all of said percentages being by weight of said solution and the relative percentages of said three solution ingredients lying within the area dened in the accompanying diagram by the line EB, the straight line BF, and the line FE, and passing an electric current therethrough of between 100 and 1,000 amperes per square foot at a hath temperature between 60 and 170 F. for a sufficient period of time to effect the polishing of said copper.

CHARLES L. FAUST.

CERTIFICATE CE CORRECTION. Patent No. 2,5LI7,C59. April I8, 191m.

CHARLES L. Emsa?.

Itis hereby Certifi ed that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 62, for I'(Cr02)"' read (CIO5); and that the said Letters l Patent should be'read with thisA correction therein that the same may confon to the record of the Case in the Patent Office.

Signed and sealed th1ys25th day of July, vA. D. 191414.

Leslie Frazer (Seal) Actngcommissioner of Patents. 

